Technical Field
[0001] The present disclosure relates to an electromagnetic-wave absorbing sheet having
flexibility and light transmittance and absorbing electromagnetic waves in a millimeter-wave
band from several tens of gigahertz (GHz) to several hundreds gigahertz (GHz) or electromagnetic
waves in a still higher frequency band.
Background Art
[0002] Electromagnetic-wave absorbing sheets for absorbing electromagnetic waves have been
used to avoid influences of leaked electromagnetic waves to be emitted to the outside
from electric circuits and influences of undesirably reflected electromagnetic waves.
[0003] Recently, researches on technologies of utilizing high frequency electromagnetic
waves, including centimeter waves having a frequency of several gigahertz (GHz), millimeter
waves having a frequency of 30 GHz to 300 GHz, and electromagnetic waves having a
still higher frequency of one terahertz (THz) as electromagnetic waves in a high frequency
band above the millimeter-wave band, have proceeded for mobile communications such
as mobile phones, wireless LAN and electric toll collection system (ETC).
[0004] There will be a growing demand, in response to such a technical trend of utilizing
electromagnetic waves of high frequencies, that electromagnetic-wave absorbers for
absorbing unnecessary electromagnetic waves and electromagnetic-wave absorbing sheets,
which are sheet-shaped electromagnetic-wave absorbers that provides higher user convenience,
can absorb electromagnetic waves in a frequency band equal to or higher than the millimeter-wave
band.
[0005] As such electromagnetic-wave absorbing sheets, electromagnetic-wave interference
type (λ/4 type) electromagnetic-wave absorbing sheets are known. In the electromagnetic-wave
interference type electromagnetic-wave absorbing sheets, an electric resistance film
is formed on one surface of a dielectric layer and an electromagnetic-wave shielding
layer that reflects electromagnetic waves is formed on the other surface of the dielectric
layer. Electromagnetic waves are absorbed by shifting the phase of reflected waves
with respect to the phase of incident waves by 1/2 wavelength to make the incident
waves and the reflected waves cancel each other out. The electromagnetic-wave interference
type electromagnetic-wave absorbing sheets can be produced easily and are lightweight
as compared with electromagnetic-wave absorbing sheets that magnetically absorb electromagnetic
waves by magnetic particles with large specific gravity, and thus are advantageous
in cost reduction.
[0006] Conventionally, in the electromagnetic-wave interference type electromagnetic-wave
absorbing sheets (electromagnetic-wave absorbers), the electric resistance film on
one surface of the dielectric layer is formed by ion plating, vacuum evaporation,
sputtering or the like, using metal oxide such as indium tin oxide (ITO), indium oxide,
stannic oxide or zinc oxide, metal nitride, or a mixture of these (see Patent Document
1, Patent Document 2).
[0007] Further, Patent Document 3 proposes an electromagnetic-wave interference type electromagnetic-wave
absorber having flame resistance and light transmittance, which includes a resistance
layer made of a transparent conductive material such as indium tin oxide (ITO) film,
a transparent dielectric layer made of glass, acrylic resin or the like, and a reflective
film made of metal such as silver, gold, copper or aluminum that is formed on the
dielectric layer.
Prior Art Documents
Patent Documents
Disclosure of Invention
Problem to be Solved by the Invention
[0009] In the conventional electromagnetic-wave absorbing sheets and electromagnetic-wave
absorbers, the electric resistance film formed on the surface of the dielectric layer
has a function of matching the surface impedance of the electromagnetic-wave absorbing
sheet to the impedance in the air to enable electromagnetic waves to easily enter
the electromagnetic-wave absorbing sheet. Therefore, it is necessary to keep the surface
electric resistance of the electric resistance film close to a vacuum permittivity
of 377 Ω/sq (sheet resistance).
[0010] On the other hand, in the electromagnetic-wave interference type electromagnetic-wave
absorbing sheets, since the dielectric layer is thinned as the frequency of electromagnetic
waves to be absorbed increases, the sheets can have high flexibility. The thin and
bendable electromagnetic-wave absorbing sheets can be attached to various places and
improve the convenience of users, but they are more likely to be bent strongly by
users. In such electromagnetic-wave absorbing sheets, the electric resistance film
formed by sputtering or the like using metal oxide cracks easily by being bent strongly,
and the surface electric resistance increases. This collapses impedance matching and
deteriorates the electromagnetic-wave absorbing properties.
[0011] Moreover, electromagnetic-wave absorbing sheets having light transmittance and flexibility
have not been realized conventionally.
[0012] It is an object of the present disclosure to provide an electromagnetic-wave absorbing
sheet that can solve the above conventional problem, specifically, to provide an electromagnetic-wave
interference type electromagnetic-wave absorbing sheet that can favorably absorb electromagnetic
waves in a desired frequency band while having high flexibility and light transmittance
and being handled easily.
Means for Solving Problem
[0013] In order to solve the above problem, an electromagnetic-wave absorbing sheet disclosed
in the present application is an electromagnetic-wave absorbing sheet having flexibility
and light transmittance, including an electric resistance film, a dielectric layer
and an electromagnetic-wave shielding layer that each have light transmittance and
that are stacked sequentially. The electric resistance film is formed of a conductive
organic polymer. The electromagnetic-wave shielding layer has an aperture ratio of
35% or more and 85% or less.
Effects of the Invention
[0014] Since the electric resistance film of the electromagnetic-wave absorbing sheet disclosed
in the present application is formed of a conductive organic polymer, the electric
resistance film does not crack even when the sheet is bent strongly, and thus the
sheet can maintain the impedance matching and keep high electromagnetic-wave absorbing
properties. Further, since the electric resistance film, the dielectric layer and
the electromagnetic-wave shielding layer each have light transmittance, the electromagnetic-wave
absorbing sheet has light transmittance. Thus, it is possible to constitute an electromagnetic-wave
absorbing sheet having high flexibility, light transmittance and favorable electromagnetic-wave
absorbing properties at low cost.
Brief Description of Drawings
[0015]
[FIG. 1] FIG. 1 is a cross-sectional view illustrating the configuration of an electromagnetic-wave
absorbing sheet of this embodiment.
[FIG. 2] FIG. 2 is a graph indicating electromagnetic-wave absorbing properties of
electromagnetic-wave absorbing sheets including electric resistance films having different
surface electric resistances.
[FIG. 3] FIG. 3 is a graph indicating electromagnetic-wave absorbing properties of
electromagnetic-wave absorbing sheets including different electromagnetic-wave shielding
layers.
[FIG. 4] FIG. 4 is a model figure illustrating the shape of the electromagnetic-wave
absorbing layer used for examining an aperture ratio.
Description of the Invention
[0016] An electromagnetic-wave absorbing sheet disclosed in the present application is an
electromagnetic-wave absorbing sheet having flexibility and light transmittance, including
an electric resistance film, a dielectric layer and an electromagnetic-wave shielding
layer that each have light transmittance and that are stacked. The electric resistance
film is formed of a conductive organic polymer. The electromagnetic-wave shielding
layer has an aperture ratio of 35% or more and 85% or less.
[0017] By doing so, the electromagnetic-wave absorbing sheet disclosed in the present application,
which is an electromagnetic-wave interference type electromagnetic-wave absorbing
sheet in which an electric resistance film, a dielectric layer and an electromagnetic-wave
shielding layer are stacked, is less likely to have a problem of cracks or the like
on the electric resistance film even when the sheet is bent strongly, thereby maintaining
the impedance matching and exhibiting high electromagnetic-wave absorbing properties.
Further, since the electric resistance film, the dielectric layer and the electromagnetic-wave
shielding layer each have light transmittance, and the electromagnetic-wave shielding
layer has a surface electric resistance high enough to function as an electromagnetic-wave
reflective layer, it is possible to constitute an electromagnetic-wave absorbing sheet
having light transmittance as a whole and not obstructing a view.
[0018] In the electromagnetic-wave absorbing sheet disclosed in the present application,
it is preferred that the electromagnetic-wave shielding layer is formed of a conductive
mesh. By doing so, the electromagnetic-wave shielding layer has a surface electric
resistance high enough to function as an electromagnetic-wave reflective layer, and
it is possible to constitute an electromagnetic-wave absorbing sheet having light
transmittance with less haze.
[0019] Further, it is preferred that a surface electric resistance of the electromagnetic-wave
shielding layer is 0.3 Ω/sq or less.
[0020] In the electromagnetic-wave absorbing sheet disclosed in the present application,
it is preferred that the electric resistance film contains poly(3,4-ethylenedioxythiophene)
(PEDOT). By doing so, it is possible to easily obtain a surface film having a desired
surface electric resistance.
[0021] In this case, it is preferred that the electric resistance film further contains
polystyrene sulfonic acid (PSS) and polyvinylidene fluoride (PVDF). The polystyrene
sulfonic acid functions as a counter anion of poly(3,4-ethylenedioxythiophene) and
stabilizes the electric resistance of the electric resistance film. Thereby, it is
possible to more easily adjust the surface electric resistance of the electric resistance
film.
[0022] Further, it is preferred that the electric resistance film contains water-soluble
polyester. By doing so, the electric resistance film itself can have high weather
resistance, and it is possible to constitute a highly reliable electromagnetic-wave
absorbing sheet with stable surface electric resistance.
[0023] Further, it is preferred that a surface electric resistance of the electric resistance
film is in a range from -15% to +20% with respect to a vacuum impedance (377 Ω). By
doing so, it is possible to obtain an electromagnetic-wave absorbing sheet that achieves
proper impedance matching to exhibit sufficiently high electromagnetic-wave absorbing
properties in practical use.
[0024] Further, in the electromagnetic-wave absorbing sheet disclosed in the present application,
it is preferred that a thickness of the dielectric layer is set so that the dielectric
layer can absorb electromagnetic waves in a high frequency band equal to or higher
than a millimeter-wave band. By doing so, it is possible to constitute an electromagnetic-wave
absorbing sheet having high flexibility and light transmittance that can absorb electromagnetic
waves in a frequency band equal to or higher than the millimeter-wave band.
[0025] Hereinafter, the electromagnetic-wave absorbing sheet disclosed in the present application
will be described with reference to the drawings.
[0026] The term "electric waves" can be understood as one type of electromagnetic waves
in a broader sense, and therefore the present specification uses the term "electromagnetic
waves" collectively. For example, electric-wave absorbers are referred to as electromagnetic-wave
absorbers in the present specification.
(Embodiment)
[0027] First, the overall configuration of an electromagnetic-wave absorbing sheet of this
embodiment will be described.
[0028] FIG. 1 is a cross-sectional view illustrating the configuration of the electromagnetic-wave
absorbing sheet of this embodiment.
[0029] FIG. 1 is illustrated for the sake of easy understanding of the configuration of
the electromagnetic-wave absorbing sheet of this embodiment, and does not faithfully
reflect the actual sizes or thicknesses of members illustrated therein.
[0030] The electromagnetic-wave absorbing sheet exemplified in this embodiment is formed
by stacking an electric resistance film 1, a dielectric layer 2, and an electromagnetic-wave
shielding layer 3. In the electromagnetic-wave absorbing sheet illustrated in FIG.
1, an adhesive layer 4 is stacked on a surface of the electromagnetic-wave shielding
layer 3 on a back side, i.e., a side in the electromagnetic-wave shielding layer 3
opposite to a side where the dielectric layer 2 is disposed. A protective layer 5
is stacked on a surface of the electric resistance film 1 on a front side, i.e., a
side in the electric resistance film 1 opposite to a side where the dielectric layer
2 is disposed.
[0031] In the electromagnetic-wave absorbing sheet of this embodiment, electromagnetic waves
11 incident upon the dielectric layer 2 are reflected at an interface between the
dielectric layer 2 and the electromagnetic-wave shielding layer 3 that is disposed
on the back side of the dielectric layer 2, and emitted to the outside as reflected
waves 12. By adjusting a thickness d of the dielectric layer 2 to 1/4 the wavelength
of incident electromagnetic waves (d = λ/4), a phase 11a of the incident waves 11
and a phase 12a of the reflected waves 12 are canceled each other out, whereby electromagnetic
waves incident upon the electromagnetic-wave absorbing sheet are absorbed.
[0032] The above formula d = λ/4 is established when the dielectric layer 2 is air (permittivity
ε = 1). When the dielectric layer 2 is a dielectric having permittivity ε
r, a formula

is established, and the thickness d of the dielectric layer 2 can be reduced by

Such a reduction in the thickness of the dielectric layer 2 can reduce the thickness
of the electromagnetic-wave absorbing sheet as a whole, whereby it is possible to
constitute an electromagnetic-wave absorbing sheet having still higher flexibility.
[0033] The electromagnetic-wave shielding layer 3, which is stacked on the back side of
the dielectric layer 2, reflects incident electromagnetic waves on the surface on
the dielectric layer 2 side, i.e., the boundary surface with the dielectric layer
2.
[0034] From the principle of electromagnetic-wave absorption in the electromagnetic-wave
interference type electromagnetic-wave absorbing sheet of this embodiment, it is necessary
that the electromagnetic-wave shielding layer 3 functions as a reflective layer that
reflects electromagnetic waves while having flexibility and light transmittance. The
electromagnetic-wave shielding layer 3 that satisfies such a demand may be a conductive
mesh constituted by conductive fibers, or a conductive grid constituted by conductive
wires such as super fine metal wires.
[0035] The electric resistance film 1 is formed on a front side of the dielectric layer
2, i.e., a side of the dielectric layer 2 from which electromagnetic waves to be absorbed
enter, which is opposite to the side where the electromagnetic-wave shielding layer
3 is stacked. The electric resistance film 1 matches impedances between the electromagnetic-wave
absorbing sheet and air.
[0036] It is important that the input impedance of the electromagnetic-wave absorbing sheet
is close to the impedance in the air of 377 Ω (vacuum impedance in practice) when
electromagnetic waves propagating through the air enter the electromagnetic-wave absorbing
sheet, in order to prevent deterioration of electromagnetic-wave absorbing properties
due to reflection or scattering of electromagnetic waves when entering the sheet.
In the electromagnetic-wave absorbing sheet of this embodiment, by forming the electric
resistance film 1 as a conductive organic polymer film, the electromagnetic-wave absorbing
sheet can have flexibility while the electric resistance film 1 does not crack even
when the electromagnetic-wave absorbing sheet is strongly bent. Thereby, the surface
electric resistance does not change, and favorable impedance matching can be maintained.
[0037] The adhesive layer 4 formed on the back side of the electromagnetic-wave shielding
layer 3 makes the electromagnetic-wave absorbing sheet easily attachable to a predetermined
position. The adhesive layer 4 can be formed easily by application of an adhesive
resin paste.
[0038] The adhesive layer 4 is not an essential member in the electromagnetic-wave absorbing
sheet of this embodiment. In an arrangement of the electromagnetic-wave absorbing
sheet to a predetermined position, a member for adhesion may be arranged on a member
on which the electromagnetic-wave absorbing sheet is to be attached, or an adhesive
may be supplied or a double-sided tape may be used between the electromagnetic-wave
absorbing sheet and an arrangement position.
[0039] The protective layer 5 is formed on a front surface of the electric resistance film
1, i.e., an uppermost surface in the electromagnetic-wave absorbing sheet on the electromagnetic-wave
incident side, to protect the electric resistance film 1.
[0040] The moisture in the air may change the surface electric resistance of the conductive
organic polymer constituting the electric resistance film 1 of the electromagnetic-wave
absorbing sheet of this embodiment. Further, since the electric resistance film 1
is made of resin, it may be damaged when a sharp member contacts the surface, or a
hard material rubs against the surface. To avoid this, it is preferable to protect
the electric resistance film 1 by covering the surface of the electric resistance
film 1 with the protective layer 5.
[0041] The protective layer 5 is not an essential member in the electromagnetic-wave absorbing
sheet of this embodiment. Depending on the material of the conductive organic polymer,
there is little concern about the change in the surface electric resistance due to
the moisture attached to the surface or the damage of the surface of the electric
resistance film 1. In this case, the electromagnetic-wave absorbing sheet can be configured
without the protective layer 5.
[0042] Moreover, the protective layer 5 may be made of a resin material such as polyethylene
terephthalate as described later. Although the resin material used as the protective
layer 5 has a certain resistance, the influence of the resistance of the protective
layer 5 on the surface electric resistance of the electromagnetic-wave absorbing sheet
can be reduced to a level having no problem in practical use by setting the thickness
of the protective layer 5 thin.
[0043] Next, members constituting the electromagnetic-wave absorbing sheet of this embodiment
will be described in detail.
[Electric resistance film]
[0044] In the electromagnetic-wave absorbing sheet of this embodiment, the electric resistance
film 1 is formed of a conductive organic polymer.
[0045] As the conductive organic polymer, a conjugated conductive organic polymer is used,
and preferable examples of which include polythiophene and derivatives thereof, and
polypyrrole and derivatives thereof.
[0046] Specific examples of the polythiophene-based conductive polymer suitably used for
the electric resistance film 1 of the electromagnetic-wave absorbing sheet of this
embodiment include poly(thiophene), poly(3-methylthiophene), poly(3-ethylthiophene),
poly(3-propylthiophene), poly(3-butylthiophene), poly(3-hexylthiophene), poly(3-heptylthiophene),
poly(3-octylthiophene), poly(3-decylthiophene), poly(3-dodecylthiophene), poly(3-octadecylthiophene),
poly(3-bromothiophene), poly(3-chlorothiophene), poly(3-iodothiophene), poly(3-cyanothiophene),
poly(3-phenylthiophene), poly(3,4-dimethylthiophene), poly(3,4-dibutylthiophene),
poly(3-hydroxythiophene), poly(3-methoxythiophene), poly(3-ethoxythiophene), poly(3-butoxythiophene),
poly(3-hexyloxythiophene), poly(3-heptyloxythiophene), poly(3-octyloxythiophene),
poly(3-decyloxythiophene), poly(3-dodecyloxythiophene), poly(3-octadecyloxythiophene),
poly(3,4-dihydroxythiophene), poly(3,4-dimethoxythiophene), poly(3,4-diethoxythiophene),
poly(3,4-dipropoxythiophene), poly(3,4-dibutoxythiophene), poly(3,4-dihexyloxythiophene),
poly(3,4-diheptyloxythiophene), poly(3,4-dioctyloxythiophene), poly(3,4-didecyloxythiophene),
poly(3,4-didodecyloxythiophene), poly(3,4-ethylenedioxythiophene), poly(3,4-propylenedioxythiophene),
poly(3,4-butenedioxythiophene), poly(3-methyl-4-methoxythiophene), poly(3-methyl-4-ethoxythiophene),
poly(3-carboxythiophene), poly(3-methyl-4-carboxythiophene), poly(3-methyl-4-carboxyethylthiophene),
and poly(3-methyl-4 -carboxybutylthiophene).
[0047] Specific examples of the polypyrrole-based conductive polymer suitably used for the
electric resistance film 1 include polypyrrole, poly(N-methylpyrrole), poly(3-methylpyrrole),
poly(3-ethylpyrrole), poly(3-n-propylpyrrole), poly(3-butylpyrrole), poly(3-octylpyrrole),
poly(3-decylpyrrole), poly(3-dodecylpyrrole), poly(3,4-dimethylpyrrole), poly(3,4-dibutylpyrrole),
poly(3-carboxypyrrole), poly(3-methyl-4-carboxypyrrole), poly(3-methyl-4-carboxyethylpyrrole),
poly(3-methyl-4-carboxybutylpyrrole), poly(3-hydroxypyrrole), poly(3-methoxypyrrole),
poly(3-ethoxypyrrole), poly(3-butoxypyrrole), poly(3-hexyloxypyrrole), poly(3-methyl-4-hexyloxypyrrole),
and poly(3-methyl-4-hexyloxypyrrole).
[0048] In addition, an organic polymer whose main chain is composed of a π conjugated system
may be used as the electric resistance film 1. Examples of such an organic polymer
include polyacetylene-based conductive polymers, polyphenylene-based conductive polymers,
polyphenylene vinylene-based conductive polymers, polyaniline-based conductive polymers,
polyacene-based conductive polymers, polythiophene vinylene-based conductive polymers,
and copolymers of these.
[0049] As the conductive organic polymer used for the electric resistance film, polyanion
may be used as a counter anion. Although not particularly limited, it is preferred
that the polyanion contains an anion group that enables the conjugated conductive
organic polymer used for the electric resistance film to cause chemical oxidation
doping. Examples of such an anion group include groups expressed by general formulae
-O-SO
3X, -OPO(OX)
2, -COOX, and -SO
3X (in each formula, X represents a hydrogen atom or an alkali metal atom). Among them,
the groups expressed by -SO
3X and -OSO
3X are particularly preferred because of their excellent doping effects on the conjugated
conductive organic polymer.
[0050] Specific examples of the polyanion include: polymers having a sulfonic group such
as polystyrene sulfonic acid, polyvinyl sulfonic acid, polyallyl sulfonic acid, polyacryl
sulfonic acid, polymethacryl sulfonic acid, poly(2-acrylamide-2-methylpropanesulfonic
acid), polyisoprene sulfonic acid, polysulfoethyl methacrylate, poly(4-sulfobutyl
methacrylate), and polymethacryloxybenzene sulfonic acid; and polymers having a carboxylic
group such as polyvinyl carboxylic acid, polystyrene carboxylic acid, polyallyl carboxylic
acid, polyacryl carboxylic acid, polymethacryl carboxylic acid, poly(2-acrylamide-2-methylpropanecarboxylic
acid), polyisoprene carboxylic acid, and polyacrylic acid. The polyanion may be a
homopolymer of one of them or a compolymer of two or more of them. One of the polyanions
may be used alone, or two or more of the polyanions may be used in combination. Among
the polyanions, polymers having a sulfonic group are preferred, and polystyrene sulfonic
acid is more preferred.
[0051] One of the conductive organic polymers may be used alone, or two or more of the conductive
organic polymers may be used in combination. Among the materials exemplified above,
polymers composed of one or two selected from polypyrrole, poly(3-methoxythiophene),
poly(3,4-ethylenedioxythiophene), poly(2-aniline sulfonic acid), and poly(3-aniline
sulfonic acid) are preferred from the viewpoint of enhancing light transmittance and
conductivity.
[0052] Particularly, it is preferable to use poly(3,4-ethylenedioxythiophene: PEDOT) and
polystyrene sulfonic acid (PSS) as a combination of the conjugated conductive organic
polymer and the polyanion.
[0053] In the electric resistance film 1 of this embodiment, a dopant may be used in combination
with the conductive organic polymer to control the electric conductivity of the conductive
organic polymer and match the input impedance of the electromagnetic-wave absorbing
sheet to the impedance in the air. Examples of the dopant include halogens such as
iodine and chlorine, Lewis acids such as BF
3 and PF
5, proton acids such as nitric acid and sulfuric acid, transition metals, alkali metals,
amino acids, nucleic acids, surfactants, pigments, chloranil, tetracyanoethylene,
and TCNQ. More specifically, it is preferable to set the surface electric resistance
of the electric resistance film 1 to about plus or minus several percent of 377 Ω.
The blending ratio between the conductive organic polymer and the dopant may be, e.g.,
conductive polymer : dopant = 1 : 2 to 1 : 4 in mass ratio.
[0054] It is preferred that the electric resistance film 1 further contains polyvinylidene
fluoride.
[0055] Polyvinylidene fluoride functions as a binder in the conductive organic polymer film
by being added to a composition for coating the conductive organic polymer, thereby
improving film formability while enhancing the adhesion with a base.
[0056] Moreover, it is preferred that the electric resistance film 1 further contains water-soluble
polyester. Since the water-soluble polyester is highly compatible with the conductive
polymer, it can fix the conductive polymer in the electric resistance film 1 by being
added to the coating composition of the conductive organic polymer constituting the
electric resistance film 1, thereby allowing the film to be more homogeneous. As a
result of the use of the water-soluble polyester, the surface electric resistance
is less likely to change even in more severe high temperature and high humidity environments,
and it is possible to maintain a state in which the impedances between the electromagnetic-wave
absorbing sheet and the air are matched.
[0057] Since the electric resistance film 1 including polyvinylidene fluoride and water-soluble
polyester can have higher weather resistance, the change in the surface electric resistance
of the electric resistance film 1 over time is reduced, and it is possible to constitute
a highly reliable electromagnetic-wave absorbing sheet that can maintain stable electromagnetic-wave
absorbing properties.
[0058] The content of the conductive organic polymer in the electric resistance film 1 is
preferably 10 mass% or more and 35 mass% or less based on the total mass of the solid
content in the composition of the electric resistance film 1. If the content of the
conductive organic polymer is less than 10 mass%, the conductivity of the electric
resistance film 1 tends to decrease. If the surface electric resistance of the electric
resistance film 1 is set within a predetermined range for impedance matching, the
thickness of the electric resistance film 1 tends to increase, which increases the
thickness of the electromagnetic-wave absorbing sheet as a whole and deteriorates
optical characteristics such as light transmittance. Meanwhile, if the content of
the conductive organic polymer exceeds 35 mass%, the coating appropriateness at the
time of coating the electric resistance film 1 tends to decrease due to the structure
of the conductive organic polymer, which makes it difficult to form a favorable electric
resistance film 1, increases the haze of the electric resistance film 1, and deteriorates
optical characteristics.
[0059] The electric resistance film 1 can be formed by applying the coating composition
that is a coating material for forming the electric resistance film onto a base and
drying it as described above.
[0060] The application method of the coating material for forming the electric resistance
film onto a base may be, e.g., bar coating, reverse coating, gravure coating, smaller
diameter gravure coating, die coating, dip coating, spin coating, slit coating, or
spray coating. Drying after the application is not particularly limited as long as
it is performed under the condition that allows a solvent component of the coating
material for forming the electric resistance film to evaporate, but it is preferably
performed at 100°C to 150°C for 5 to 60 minutes. If a solvent remains in the electric
resistance film 1, the strength tends to deteriorate. The drying method may be, e.g.,
hot-air drying, heat drying, vacuum drying, or natural drying. The electric resistance
film 1 may be formed by curing the coated film by irradiation with UV light (ultraviolet
light) or EB (electron beams) as needed.
[0061] The base to be used for forming the electric resistance film 1 is not particularly
limited, but it is preferably a transparent base having light transmittance. Various
materials such as resin, rubber, glass, and ceramics can be used as the material of
the transparent base.
[0062] By forming the electric resistance film 1 having a surface electric resistance of
377 Ω/sq using the conductive organic polymer, the input impedance of the electromagnetic-wave
absorbing sheet of this embodiment can be matched to the impedance in the air. Thereby,
reflection or scattering of electromagnetic waves on the surface of the electromagnetic-wave
absorbing sheet can be reduced, and more favorable electromagnetic-wave absorbing
properties can be obtained.
[Dielectric layer]
[0063] The dielectric layer 2 of the electromagnetic-wave absorbing sheet of this embodiment
can be formed of a dielectric such as polyvinylidene fluoride, polyester resin, glass,
transparent silicone rubber, or transparent OCA or OCR. The dielectric layer 2 may
be a single layer formed of one material, or a stack of two or more layers formed
of the same material or different materials. The formation method of the dielectric
layer 2 may be, e.g., an application method, press molding, or extrusion molding.
[0064] As described above, the electromagnetic-wave absorbing sheet of this embodiment is
an electromagnetic-wave interference type (λ/4 type) electromagnetic-wave absorbing
sheet that absorbs electromagnetic waves by shifting the phase of electromagnetic
waves incident upon the electromagnetic-wave absorbing sheet and the phase of reflected
waves reflected by the electromagnetic-wave shielding layer by 1/2 wavelength to make
the incident waves and the reflected waves cancel each other out. Therefore, the thickness
of the dielectric layer (d in FIG. 1) is determined corresponding to the wavelength
of electromagnetic waves to be absorbed.
[0065] Incidentally, the formula d = λ/4 is established when an interspace between the electric
resistance film 1 and the electromagnetic-wave shielding layer 3 is a space, i.e.,
the dielectric layer 2 is constituted by air. When the dielectric layer 2 is formed
of a material having permittivity ε
r, the thickness d becomes

By using as a material constituting the dielectric layer 2 a material having large
permittivity, the thickness d of the dielectric layer 2 can be reduced by

, and the thickness of the electromagnetic-wave absorbing sheet as a whole can be
reduced. Since the electromagnetic-wave absorbing sheet of this embodiment has flexibility,
it is more preferred that the dielectric layer 2 constituting the electromagnetic-wave
absorbing sheet and the electromagnetic-wave absorbing sheet itself are as thin as
possible to make the sheet bent more easily. Further, considering that the electromagnetic-wave
absorbing sheet of this embodiment is often to be attached via the adhesive layer
4 (described later) or the like to a member whose electromagnetic wave leakage is
desired to be prevented, it is preferred that the electromagnetic-wave absorbing sheet
is thin to easily conform to the shape of an attachment part and lightened further.
[0066] As compared with the case of arranging the electric resistance film 1 at a position
λ/4 away from the electromagnetic-wave shielding layer 3, the thickness d can be d
= λ / (

) by using the dielectric layer 2 having permittivity ε
r between the electromagnetic-wave shielding layer 3 and the electric resistance film
1, whereby the thickness of the dielectric layer 2 can be reduced. In this manner,
by adjusting the permittivity ε
r and the thickness of the dielectric layer 2, it is possible to control the wavelength
of electromagnetic waves to be absorbed by the electromagnetic-wave absorbing sheet
including the dielectric layer 2.
[Electromagnetic-wave shielding layer]
[0067] The electromagnetic-wave shielding layer 3 of the electromagnetic-wave absorbing
sheet of this embodiment reflects electromagnetic waves incident from the surface
film 1, which is disposed opposite to the electromagnetic-wave shielding layer 3 via
the dielectric layer 2 in the electromagnetic-wave absorbing sheet.
[0068] It is necessary that the electromagnetic-wave shielding layer 3 has flexibility so
that it bends following at least the electric resistance film 1 and the dielectric
layer 2 while having light transmittance.
[0069] The electromagnetic-wave shielding layer 3 that satisfies such a demand may be a
conductive mesh constituted by conductive fibers. In one example, the conductive mesh
can be formed by depositing a metal on a mesh woven from polyester monofilaments to
impart conductivity to the mesh. The metal may be highly conductive metal such as
copper or silver. Further, in order to reduce reflection by the metal film covering
the mesh surface, a product in which a black antireflective layer is further provided
on the outer side of the metal film is also on the market.
[0070] In addition to the above, the electromagnetic-wave shielding layer 3 may be a conductive
grid constituted by fine metal wires (e.g., copper wires) having a diameter of several
tens to several hundreds µm, which are arranged horizontally and vertically.
[0071] In order to obtain flexibility and light transmittance, the thickness of the electromagnetic-wave
shielding layer 3 constituted by the conductive mesh or conductive grid is set to
minimal within a range in which the electromagnetic-wave shielding layer 3 can have
a desired surface electric resistance. If the fibers of the conductive mesh or the
wires of the conductive grid are damaged or cut, it becomes difficult to obtain a
desired surface electric resistance. To avoid this, a reinforcing layer and a protective
layer made of resin having light transmittance may be formed on the back side of the
conductive grid so that the electromagnetic-wave shielding layer 3 is configured as
a stack of an electromagnetic-wave reflecting part made of a conductive material and
a film constituting part made of resin.
[Adhesive layer]
[0072] By providing the adhesive layer 4 in the electromagnetic-wave absorbing sheet of
this embodiment, the electromagnetic-wave absorbing sheet as a stack of the electric
resistance film 1, the dielectric layer 2 and the electromagnetic-wave shielding layer
3 can be attached to a desired position such as an inner surface of a housing that
contains an electric circuit, or an inner surface or outer surface of an electric
device. Specifically, since the electromagnetic-wave absorbing sheet of this embodiment
has flexibility, it can be attached easily on a curved surface (bent surface). Thus,
the adhesive layer 4 provided on the back surface of the sheet improves the handleability
of the electromagnetic-wave absorbing sheet.
[0073] The adhesive layer 4 may be formed of a known material generally used as an adhesive
layer such as an adhesive tape, and specific examples of which include an acrylic
adhesive, a rubber adhesive, and a silicone adhesive. A tackifier or crosslinking
agent may be used to adjust the tackiness with respect to an adherend and to reduce
adhesive residues. The tackiness with respect to an adherend is preferably 5 N/10
mm to 12 N/10 mm. If the tackiness is smaller than 5 N/10 mm, the electromagnetic-wave
absorbing sheet may be easily peeled off or displaced from an adherend. If the tackiness
is larger than 12 N/10 mm, the electromagnetic-wave absorbing sheet is difficult to
be peeled off from an adherend.
[0074] The thickness of the adhesive layer 4 is preferably 20 µm to 100 µm. If the adhesive
layer 4 is thinner than 20 µm, the tackiness is low and the electromagnetic-wave absorbing
sheet may be easily peeled off or displaced from an adherend. If the adhesive layer
4 is thicker than 100 µm, the electromagnetic-wave absorbing sheet is difficult to
be peeled off from an adherend. If the cohesion of the adhesive layer is low, an adhesive
may remain on an adherend when the electromagnetic-wave absorbing sheet is peeled
off from the adherend, and the flexibility of the electromagnetic-wave absorbing sheet
as a whole may decrease.
[0075] The adhesive layer 4 to be used in the electromagnetic-wave absorbing sheet of this
embodiment may be an adhesive layer 4 that makes the electromagnetic-wave absorbing
sheet unpeelably attached to an adherend, or an adhesive layer 4 that makes the electromagnetic-wave
absorbing sheet peelably attached to an adherend. Further, as described above, the
adhesive layer 4 is not essential in the electromagnetic-wave absorbing sheet of this
embodiment, and various conventional adhesion methods can be adopted to attach the
electromagnetic-wave absorbing sheet to a desired member.
[Protective layer]
[0076] In the electromagnetic-wave absorbing sheet of this embodiment, the protective layer
5 may be provided on a surface of the electric resistance film 1 on the electromagnetic-wave
incident side.
[0077] In the electromagnetic-wave absorbing sheet of this embodiment, the surface electric
resistance of the conductive organic polymer used as the electric resistance film
1 may change due to humidity in the air. By providing the protective layer 5 on the
surface of the electric resistance film 1, the influence of humidity can be reduced,
and the electromagnetic-wave absorbing properties obtained by impedance matching can
be effectively prevented from deteriorating.
[0078] In one example, the protective layer 5 in the electromagnetic-wave absorbing sheet
of this embodiment can be polyethylene terephthalate having a thickness of 25 µm,
which is attached on the surface of the electric resistance film 1 using an adhesive
of a resin material.
[0079] By configuring the protective layer 5 to cover the entire surface of the electric
resistance film 1, it is possible to prevent the influence of moisture in the air
on the electric resistance film 1. Further, it is considered that the surface electric
resistance of the protective layer 5 formed as a resin film may affect the surface
electric resistance of the electric resistance film 1 as a member connected in parallel
with the electric resistance film 1. Because of this, if the protective layer 5 is
not too thick, the influence given to the input impedance of the electromagnetic-wave
absorbing sheet will be very little. It is also possible to set the surface electric
resistance of the electric resistance film 1 to a more suitable value by taking into
consideration the influence of the surface electric resistance of the protective layer
5 as an input impedance of the electromagnetic-wave absorbing sheet.
[0080] It is preferred that the thickness of the protective layer 5 is as thin as possible
within a range that can protect the electric resistance film 1. Specifically, the
thickness of the protective layer 5 is preferably 150 µm or less, and more preferably
100 µm or less. If the thickness of the protective layer exceeds 150 µm, electromagnetic-wave
absorbency may decrease and the electromagnetic-wave absorption amount may be lower
than 20 dB, and the thickness of the electromagnetic-wave absorbing sheet as a whole
increases and the flexibility decreases.
[Examples]
[0081] Electromagnetic-wave absorbing sheets of this embodiment were actually produced to
measure various properties. The following describes the results.
<Weather resistance of electric resistance film>
[0082] Five each of the following two kinds of electromagnetic-wave absorbing sheets were
produced by differentiating the components of electric resistance film liquids (liquids
for forming electric resistance film).
(Sheet 1)
[0083] An electric resistance film liquid of Sheet 1 was prepared by adding and mixing the
following components.
(1) Conductive polymer dispersing element |
36.7 parts |
Conductive polymer (PEDOT-PSS) manufactured by Heraeus Holding: |
PH-100 (trade name), |
Solid content concentration: 1.2 mass% |
(2) PVDF dispersion |
5.6 parts |
LATEX 32 (trade name) manufactured by Arkema S.A., |
Solid content concentration: 20 mass%, Solvent: Water |
(3) Water-soluble polyester aqueous solution |
0.6 parts |
PLAS COAT Z561 (trade name) manufactured by GOO CHEMICAL CO., LTD., |
Solid content concentration: 25 mass% |
(4) Organic solvent (dimethylsulfoxide) |
9.9 parts |
(5) Water-soluble solvent (ethanol) |
30.0 parts |
(6) Water |
17.2 parts |
(Sheet 2)
[0084] An electric resistance film liquid of Sheet 2 was prepared by adding and mixing the
following components.
(1) Conductive polymer dispersing element |
33.7 parts |
Conductive polymer (PEDOT-PSS) manufactured by Heraeus Holding: |
PH-1000 (trade name), Solid content concentration: 1.2 mass% |
(2) PVDF dispersion |
5.1 parts |
LATEX 32 (trade name) manufactured by Arkema S.A., |
Solid content concentration: 20 mass%, Solvent: Water |
(3) Organic solvent (dimethylsulfoxide) |
9.5 parts |
(4) Water-soluble solvent (n-propyl alcohol) |
36.0 parts |
(5) Water |
15.7 parts |
[0085] Each electric resistance film was formed by applying the electric resistance film
liquid in the proportion described above onto a polyethylene terephthalate sheet (25
µm thick, base) by bar coating, in an amount so that the thickness after drying would
be about 120 nm, and the applied liquid was heated at 150°C for five minutes. The
surface electric resistances of the electric resistance films thus produced were all
377 Ω/sq.
[0086] An urethane rubber having a thickness of 400 µm was used as the dielectric layer,
and an aluminum foil having a thickness of 15 µm was used as the electromagnetic-wave
shielding layer. The electric resistance film, the dielectric layer, and the aluminum
foil were stacked in close contact with each other and attached to each other using
an adhesive.
(Test conditions)
[0087] The initial surface electric resistances of Sheet 1 (n = 5) and Sheet 2 (n = 5) produced
above were measured. Next, all of the electromagnetic-wave absorbing sheets were placed
in a thermo-hygrostat and stored for 500 hours at 60°C under a relative humidity of
90%. Subsequently, the surface electric resistances of the electric resistance films
of the electromagnetic-wave absorbing sheets after storage were measured. Then, surface
electric resistance change rates were calculated based on the formula below, and an
average of the surface electric resistance change rates of the five (n = 5) electromagnetic-wave
absorbing sheets was calculated.

[0088] As a result of the above measurement, the average of the surface electric resistance
change rates of the five (n = 5) electromagnetic-wave absorbing sheets was 8% for
Sheet 1 and 18% for Sheet 2. The surface electric resistance change rate of 8% of
Sheet 1 corresponds to about 30 Ω with respect to 377 Ω, which is judged that Sheet
1 has high stability in practical use, considering the severe weather resistance test
conditions. The surface electric resistance change rate of 18% of Sheet 2 corresponds
to about 68 Ω with respect to 377Ω, which is judged that Sheet 2 has enough stability
in practical use.
[0089] The results of the weather resistance test using Sheet 1 and Sheet 2 indicate that
the hygroscopicity of the electric resistance film is lowered by adding the water-soluble
polyester aqueous solution to the electric resistance film, and thereby it is possible
to constitute an electromagnetic-wave absorbing sheet having stable electromagnetic-wave
absorbing properties with less surface electric resistance change.
<Effects of impedance matching>
[0090] Next, the change in the electromagnetic-wave absorbing properties depending on the
surface electric resistance of the electric resistance film in the electromagnetic-wave
absorbing sheet of this embodiment was examined by actually producing electromagnetic-wave
absorbing sheets (Sheet 3 to Sheet 6) including electric resistance films with different
surface electric resistances.
(Production of sheets)
[0091] Electromagnetic-wave absorbing sheets were produced in the following manner. Electric
resistance films having different thicknesses were formed by applying the electric
resistance film liquid of Sheet 1 onto a 300-µm-thick polyethylene terephthalate (base)
in different thicknesses by bar coating, followed by heating at 150°C for five minutes.
Then, a 250-µm-thick polyethylene terephthalate sheet was attached to a surface of
the polyethylene terephthalate (base) using an adhesive on a side opposite to the
side where the electric resistance film layer was formed. As a result, the dielectric
layer 2 of polyethylene terephthalate having a thickness of 550 µm was formed. A 15-µm-thick
aluminum foil was used as the electromagnetic-wave shielding layer 3. The center frequency
of electromagnetic waves to be absorbed by each of the electromagnetic-wave absorbing
sheets thus produced was 76 GHz.
[0092] The thicknesses and surface electric resistances of the electric resistance film
layers after drying of the electromagnetic-wave absorbing sheets were as below.
(Sheet 3) Electric resistance film layer, thickness: 140 nm, surface electric resistance:
320 Ω/sq
(Sheet 4) Electric resistance film layer, thickness: 92 nm, surface electric resistance:
452 Ω/sq
(Sheet 5) Electric resistance film layer, thickness: 15 nm, surface electric resistance:
302 Ω/sq
(Sheet 6) Electric resistance film layer, thickness: 88 nm, surface electric resistance:
471 Ω/sq
(Measurement of electromagnetic-wave absorbing properties)
[0093] The electromagnetic-wave absorbing properties of Sheet 3 to Sheet 6 produced above
as well as Sheet 1, which is the electromagnetic-wave absorbing sheet including the
electric resistance film having a surface electric resistance of 377 Ω/sq (the same
as the impedance in the air), were measured in accordance with a free space method.
Specifically, a free space measuring device manufactured by KEYCOM Corporation and
a vector network analyzer MS4647 B (trade name) manufactured by ANRITSU CORPORATION
were used to determine, as a voltage, a ratio between the intensity of incident waves
and the intensity of reflected waves at the time of irradiating each of the electromagnetic-wave
absorbing sheets with electromagnetic waves.
[0094] FIG. 2 indicates electromagnetic-wave absorbing properties of each of the electromagnetic-wave
absorbing sheets measured in the above-described manner. In FIG. 2, the attenuation
amount of the intensity of reflected waves with respect to the intensity of incident
waves is expressed in dB.
[0095] In FIG. 2, reference numeral 21 indicates the electromagnetic-wave absorbing properties
of Sheet 1, reference numeral 22 indicates the electromagnetic-wave absorbing properties
of Sheet 3, reference numeral 23 indicates the electromagnetic-wave absorbing properties
of Sheet 4, reference numeral 24 indicates the electromagnetic-wave absorbing properties
of Sheet 5, and reference numeral 25 indicates the electromagnetic-wave absorbing
properties of Sheet 6.
[0096] FIG. 2 indicates that Sheet 1, including the electric resistance film having a surface
electric resistance of 377 Ω/sq (matched the impedance in the air (vacuum)) and achieving
extremely favorable impedance matching, resulted in extremely high attenuation amount
of about 42 dB with respect to electromagnetic waves of 76 GHz.
[0097] Sheet 3, including the electric resistance film having a surface electric resistance
of 320 Ω/sq (shifted by -15% from the vacuum impedance (377Ω)), resulted in the electromagnetic-wave
attenuation amount of about 22 dB at 76 GHz, and Sheet 4, including the electric resistance
film having a surface electric resistance of 452 Ω/sq (shifted by +20% from the vacuum
impedance), resulted in the electromagnetic-wave attenuation amount of about 21 dB
at 76 GHz. Both of these sheets exceeded the electromagnetic-wave attenuation amount
of 20 dB (attenuation rate: 99%) and exhibited favorable electromagnetic-wave absorbing
properties.
[0098] Meanwhile, Sheet 5, including the electric resistance film having a surface electric
resistance of 302 Ω/sq (shifted by -20% from the vacuum impedance (377 Ω)), and Sheet
6, including the electric resistance film having a surface electric resistance of
471 Ω/sq (shifted by +25% from the vacuum impedance), both resulted in the electromagnetic-wave
attenuation amount of about 19 dB at 76 GHz. It is considered that the attenuation
amount of about 20 dB or more is practical electromagnetic-wave absorbing properties
as the electromagnetic-wave absorbing sheet. By setting the surface electric resistance
of the electric resistance film within a range from -15% to +20% with respect to the
vacuum impedance, it is possible to obtain an electromagnetic-wave absorbing sheet
having favorable electromagnetic-wave absorbing properties.
[Electromagnetic-wave shielding layer]
[0099] Next, an electromagnetic-wave shielding layer having flexibility and light transmittance
was examined.
[0100] An electric resistance film having an electric resistance of 377 Ω/sq was produced
based on the production method of Sheet 1.
[0101] Specifically, each electric resistance film was formed by applying the electric resistance
film liquid onto polyethylene terephthalate (10 µm thick, base) by bar coating, and
the applied liquid was heated at 150°C for five minutes. Then, a dielectric layer
was formed using a 550-µm-thick transparent silicone rubber on a surface of the polyethylene
terephthalate (base) on a side opposite to the side where the electric resistance
film layer was formed.
[0102] An electromagnetic-wave shielding layer of Sheet 7 (Example 1) was formed using a
conductive mesh Su-4X-27035 (trade name) manufactured by SEIREN Co., Ltd. An electromagnetic-wave
shielding layer of Sheet 8 (Example 2) was formed using a conductive mesh Su-4G-9027
(trade name) manufactured by SEIREN Co., Ltd.
[0103] An electromagnetic-wave shielding layer of Sheet 9 (Comparative Example) was formed
using a transparent conductive film PURE-OPT RN 3000 (trade name) manufactured by
FUJIMORI KOGYO CO., LTD.
[0104] The electric characteristics and the optical characteristics of the electromagnetic-wave
shielding layers in the electromagnetic-wave absorbing sheets were as below.
(Sheet 7) Surface electric resistance: 0.04 Ω/sq, total light transmittance: 30%,
aperture ratio: 38%
(Sheet 8) Surface electric resistance: 0.11 Ω/sq, total light transmittance: 66%,
aperture ratio: 82%
(Sheet 9) Surface electric resistance: 0.40 Ω/sq, total light transmittance: 77% or
more
[0105] The total light transmittance, the haze value and the electromagnetic-wave absorbing
properties of each of the three electromagnetic-wave absorbing sheets thus produced
were measured.
[0106] The total light transmittance and the haze value were measured using a Haze Meter
NDH2000 (trade name) manufactured by NIPPON DENSHOKU INDUSTRIES Co., Ltd., in accordance
with JIS K7105. A Light C was used as a light source.
[0107] The electromagnetic-wave absorbing properties were measured in accordance with the
free space method described above. Specifically, a free space measuring device manufactured
by KEYCOM Corporation and a vector network analyzer MS4647 B (trade name) manufactured
by ANRITSU CORPORATION were used to determine, as a voltage, a ratio between the intensity
of incident waves and the intensity of reflected waves at the time of irradiating
each of the electromagnetic-wave absorbing sheets with electromagnetic waves.
[0108] FIG. 3 indicates electromagnetic-wave absorbing properties of each of the electromagnetic-wave
absorbing sheets measured in the above-described manner. In FIG. 3, the attenuation
amount of the intensity of reflected waves with respect to the intensity of incident
waves is expressed in dB.
[0109] In FIG. 3, reference numeral 31 indicates the electromagnetic-wave absorbing properties
of Sheet 7, reference numeral 32 indicates the electromagnetic-wave absorbing properties
of Sheet 8, and reference numeral 33 indicates the electromagnetic-wave absorbing
properties of Sheet 9.
[0110] The optical characteristics of each of the electromagnetic-wave absorbing sheets
were as follows: Sheet 7 had a total light transmittance of 30% and a haze value of
40; Sheet 8 had a total light transmittance of 66% and a haze value of 7; and Sheet
9 had a total light transmittance of 77% and a haze value of 8.
[0111] Here, a relationship between the aperture ratio and the surface electric resistance
of the electromagnetic-wave shielding layer was simulated.
[0112] FIG. 4 is a model figure illustrating the shape of the electromagnetic-wave absorbing
layer used for the examination.
[0113] As illustrated in FIG. 4, it was assumed that the electromagnetic-wave absorbing
layer was a grid metal mesh in which metal wires extend in vertical and horizontal
directions. A change in the aperture ratio in accordance with the change in a pitch
P of the metal wires and the conductance as the metal layer were calculated, by taking
one square constituted by the metal wires (conductive material) as an inductance element
(coil) as a loop.
[0114] More specifically, it was assumed that metal wires 27 µm in diameter were used. The
aperture ratio of the electromagnetic-wave absorbing layer at this time is expressed
by Formula (1) below, based on Pitch P = Wire diameter L + Space S between wires.
[Numerical Formula 1]

[0115] When the attenuation amount of electromagnetic waves incident upon the platy electromagnetic-wave
absorbing layer is expressed in dB as a shielding SE, it is expressed by Formula (2)
below, where Z
0 represents the input and out impedance of the metal plate, σ(Ω
-1·m
-1) represents the conductance of the metal plate, and d(m) represents the plate thickness.
[Numerical Formula 2]

[0116] Here, when each square of the metal mesh is regarded as a coil, and the resistance
R = 1 / (σ·d) as the metal plate is replaced with jωL, the above Formula (2) can be
converted to Formula (3) below.
[Numerical Formula 3]

[0117] Since ω is 2nL (ω = 2πL), the electromagnetic-waves shielding SE can be expressed
by Formula (4) below.
[Numerical Formula 4]

[0118] The aperture ratio (Formula (1)) and the shielding SE were measured by changing the
pitch P of the wires constituting the metal mesh from 30 µm to 500 µm. It was found
from this measurement that, in order to achieve the shielding SE of 20 dB, which corresponds
to the attenuation amount of 99.9%, at electromagnetic wave frequencies of 60 to 90
GHz, the upper limit of the pitch P of the metal wires was substantially 170 µm as
indicated in Table 1 below. At this time, the aperture ratio was 75%, and the total
light transmittance considering wire bending was 60%.
[0119] Meanwhile, in order to achieve an electromagnetic-wave absorbing sheet having light
transmittance, it is considered that the electromagnetic-wave absorbing layer is required
to have a total light transmittance of 30% or more. The wire pitch P to achieve this
was 50 µm, and the aperture ratio at this time was 35% and the shielding SE indicating
the electromagnetic-wave attenuation amount was 45 dB.
[Table 1]
Frequency |
60-90 GHz |
Pitch P of metal wires |
170 µm |
50 µm |
Aperture ratio |
75% |
35% |
Transmittance |
60% |
30% |
Shielding SE |
21.2 dB |
45.0 dB |
[0120] The above examination results of the electromagnetic-wave shielding effects in the
electromagnetic-wave shielding layers and the optical characteristics of the electromagnetic-wave
shielding layers of Sheets 7 and 8 indicate that the aperture ratio of 35% or more
and 85% or less is a preferable condition in the case of using a conductive mesh or
a conductive grid. The aperture ratio of 35% or more and 85% or less is a more preferable
condition.
[0121] It is judged also from the result of Sheet 9 that the surface electric resistance
is preferably 0.3 Ω/sq or less, and more preferably 0.11 Ω/sq or less to obtain favorable
electromagnetic-wave reflecting properties as the electromagnetic-wave absorbing layer.
[Effect of protective layer]
[0122] Next, an effect obtained by stacking the protective layer on the surface of the electric
resistance film was examined.
[0123] As an electromagnetic-wave absorbing sheet, Sheet 10 was formed using Sheet 1 described
above and a 25-µm-thick polyethylene terephthalate sheet with an adhesive layer as
a as a protective layer, and attaching the polyethylene terephthalate sheet to the
surface of the electric resistance film.
[0124] Two each of Sheet 1 and Sheet 10 were prepared. The four electromagnetic-wave absorbing
sheets in total were each subjected to a dry-wiping sliding test to measure the presence
or absence of abrasion on the surface sheet member and the change in the surface electric
resistance. The dry-wiping sliding test was performed using a HEIDON sliding test
machine set with a white flannel cloth under the following conditions: weight: 2000
g, sliding rate: 4500 mm/min, sliding width: 25 mm, and sliding frequency: 1000 passes
(about 10 minutes).
[0125] The electromagnetic-wave absorbing sheets after the test were observed. No abrasion
was found visually on any of the two each of Sheet 1 and Sheet 10. As to the surface
electric resistances of the electric resistance films of the electromagnetic-wave
absorbing sheets, the two Sheets 10 provided with the protective layer had no change,
whereas the two Sheets 1 not provided with the protective layer increased the surface
electric resistance by 16% and 10%, respectively. The reason for this is considered
to be that the electric resistance films of the electromagnetic-wave absorbing sheets
not provided with the protective layer were scraped during the sliding test, and the
thicknesses reduced and the surface electric resistances increased.
[0126] The above results confirmed the following. The change in the surface electric resistance
of the electric resistance film collapses the impedance matching and deteriorates
the electromagnetic-wave absorbing properties. By providing the protective layer,
it is possible to reduce the change in the thickness of the electric resistance film
due to mechanical factors and configure an electromagnetic-wave absorbing sheet having
stable electromagnetic-wave absorbing properties.
[Confirmation of flexibility]
[0127] Next, it was confirmed that, by using the conductive organic polymer as the electric
resistance film, the electromagnetic-wave absorbing sheet of this embodiment can have
flexibility.
[0128] Sheet 11 was produced as a comparative example. An electric resistance film of Sheet
11 having a surface electric resistance of 370 Ω/sq was formed by sputtering indium
tin oxide (ITO). A dielectric layer and an electromagnetic-wave shielding layer of
Sheet 11 were the same as those of Sheet 1.
[0129] Sheet 1 and Sheet 11 were each cut into a size of 5 x 10 cm, and their initial surface
electric resistances were measured. Next, the sheets were placed on horizontally arranged
six aluminum cylindrical rods (mandrels) 10 mm, 8 mm, 6 mm, 4 mm, 2 mm and 0.5 mm
in diameter so that the electric resistance films would face upward. A weight of 300
g was attached to both ends of the sheets, and this state was maintained for 30 seconds.
The both ends of the sheets were pulled downward with the center of the sheets being
bent. Then, the surface electric resistances of the electromagnetic-wave absorbing
sheets were measured.
[0130] Table 2 below indicates the measurement results.
[Table 2]
Diameter of cylindrical rod |
0.5 mm |
2 mm |
4 mm |
6 mm |
8 mm |
10 mm |
Electric resistance film |
PEDOT |
PEDOT |
PEDOT |
PEDOT |
PEDOT |
PEDOT |
Surface electric resistance of electric resistance film after wounded around rod |
370 Ω |
370 Ω |
370 Ω |
370 Ω |
370 Ω |
370 Ω |
Surface condition |
No change |
No change |
No change |
No change |
No change |
No change |
Electric resistance film |
ITO |
ITO |
ITO |
ITO |
ITO |
ITO |
Surface electric resistance of electric resistance film after wounded around rod |
∞ |
∞ |
|
750 Ω/sq |
|
370 Ω |
Surface condition |
More cracks |
More cracks |
|
With cracks |
|
No change |
[0131] As a result, in the case of the aluminum cylindrical rod 10 mm in diameter, the surface
electric resistances of the electric resistance films of Sheet 1 and Sheet 11 did
not change. In the case of the aluminum cylindrical rod 6 mm in diameter, the surface
electric resistance of the electric resistance film of Sheet 1 did not change, but
the surface electric resistance of the electric resistance film of Sheet 11 increased
to 750 Ω/sq, which is about twice the initial surface electric resistance. In the
cases of the aluminum cylindrical rods 2 mm and 0.5 mm in diameter, the surface electric
resistance of the electric resistance film of Sheet 1 did not change, but the surface
electric resistance of the electric resistance film of Sheet 11 became infinite and
the film could no longer be used as the electric resistance film.
[0132] A microscope was used to observe the surface conditions of the electromagnetic-wave
absorbing sheets wounded around the aluminum cylindrical rod 6 mm in diameter. No
change was observed on the surface of Sheet 1, but cracks appeared on the surface
of Sheet 11. Further, the surface conditions of the electromagnetic-wave absorbing
sheets wounded around the aluminum cylindrical rod 0.5 mm in diameter were observed
by the microscope. No change was observed on the surface of Sheet 1, but more cracks
appeared on the surface of Sheet 11 than the surface of Sheet 11 wounded around the
aluminum cylindrical rod 6 mm in diameter.
[0133] The above result confirmed that, by using the conductive organic polymer as the electric
resistance film in the electromagnetic-wave absorbing sheet of this embodiment, the
flexibility of the sheet improves and the electromagnetic-wave absorbing properties
can be maintained even when a load that causes the sheet to strongly bend with a small
diameter, is applied to the sheet.
[0134] As described above, in the electromagnetic-wave absorbing sheet of this embodiment,
by constituting the electric resistance film to be arranged on the surface on the
electromagnetic-wave incident side using the conductive organic polymer, electromagnetic-wave
absorbing properties can be maintained even when the electromagnetic-wave absorbing
sheet is strongly bent. Thus, it is possible to constitute an electromagnetic-wave
absorbing sheet capable of exhibiting stable and high electromagnetic-wave absorbing
properties while having flexibility and light transmittance. For example, the electromagnetic-wave
absorbing sheet can be suitably used in a situation in which it is necessary to absorb
undesired electromagnetic waves to avoid transmission of undesired electromagnetic
waves while allowing a user to observe the conditions inside or outside the sheet,
such as a curtain to create an electromagnetic-wave shielded room.
Industrial Applicability
[0135] The electromagnetic-wave absorbing sheet disclosed in the present application is
useful as an electromagnetic-wave absorbing sheet that can stably absorb electromagnetic
waves in a high frequency band equal to or higher than the millimeter-wave band while
having flexibility and light transmittance.
Description of Reference Numerals
[0136]
- 1
- Electric resistance film
- 2
- Dielectric layer
- 3
- Electromagnetic-wave shielding layer
- 4
- Adhesive layer
- 5
- Protective layer